Zoning and recrystallization of phengitic micas: implications for metamorphic equilibration

White micas (phengites) in the metasediments of the Scottish Dalradian display a large range of compositions within single samples. The variations in the composition of these phengites are strongly controlled by their structural age, with early fabrics containing a paragonite-poor, celadonite-rich phengite whereas in later fabrics the micas are generally paragonite-rich and celadonite-poor. Retrograde phengite growth, identified using back scattered electron imaging, occurs as celadonite-rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica-plagioclase grain boundaries. The presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient. It is proposed that diffusion-controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible. This closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism. Many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation/recrystallization controlled equilibria.     

Evaluation of thermobarometers for garnet peridotites

The accuracy and precision of a large number of combinations of geothermometers and geo-barometers for garnet lherzolites have been evaluated with a suite of well-equilibrated xenoliths from kimberlites of northern Lesotho. Accuracy was tested by comparison of P-T estimates for a diamond-bearing and a graphite-bearing xenolith with the experimentally determined diamond-graphite univariant curve and by comparison of P-T estimates for phlogopite-bearing xenoliths to the high-temperature stability limit of phlogopite (Eggler and Wendlandt, 1979). Precision was evaluated by measuring the scatter of P-T estimates for each of four xenoliths from a wide range of P and T when many point analyses of the constituent minerals are used for P-T estimation. A thermobarometer composed of the uncorrected diopside-enstatite miscibility gap of Lindsley and Dixon (1976), combined with the uncorrected isopleths for aluminum in enstatite coexisting with pyrope of MacGregor (1974), is most satisfactory. Correction schemes such as those of Wells (1977) and Wood (1974) will ultimately provide a better means of P-T estimation, but at the present stage of development they serve to decrease precision without a demonstrable increase in accuracy. Thermometers based on $\text{Fe}{}^{\mathrm{2+}}\text{Mg}$ exchange reactions are imprecise because of variable and unknown $\text{Fe}{}^{\mathrm{2+}}\text{Fe}{}^{\mathrm{3+}}$ in minerals and xenoliths. The inflection observed in the northern Lesotho paleogeotherm cannot be an artifact of the method of temperature estimation.     

云母的命名

根据容许的化学成分范围，给出了纯云母类（true micas)，脆云母类（brittle micas)和层间阳离子亏损的云母类（interlayer-cation-deficient micas)的端元矿物和矿物种，概括介绍了利用现有的各种化学分析数据计算云母晶体化学式的方法，以及描述不常见化学元素代换或多型堆积的一系列修饰词和后缀，列出了云母的同义词和变种、定名有误的物质，以及过去使用或误用的云母名称。     

Chemistry of micas from kimberlites and xenoliths—II. Primary- and secondary-textured micas from peridotite xenoliths

Micas from coarse granular Iherzolites in S. African kimberlites may be separated into two groups; those showing primary textural relationships with coexisting silicates and those with secondary, alteration relationships with other silicates. Primary-textured micas form a tight cluster with a mean composition from 10 coarse garnet lherzolites of: SiO₂ 41.0, TiO₂ 0.18, Al₂O₃ 13.5, Cr₂O₃ 0.82, total Fe as FeO 2.60, MnO 0.02, MgO 26.0, NiO 0.22, CaO 0.01, BaO 0.29, Na₂O 0.31, K₂O 10.0, Rb₂O 0.028, Cl 0.08, F 0.43 wt%. Primary-textured micas in aggregates with clinopyroxene have higher TiO₂ and four specimens which look similar to the primary group but have textural ambiguities have still higher TiO₂Micas with secondary textures have wide ranges of composition which may be correlated with details of the textural parageneses. Micas from kelyphitic rims around garnets tend to be Cr-rich while those from veinlets are Cr-poor. Both groups tend to have higher FeO and TiO₂ than the primary group. Micas produced by alteration of, or filling veinlets through, orthopyroxene have a wide compositional range which overlaps that of the primary-textured micas, especially for harzburgite specimens.The primary-textured micas show a positive correlation with coexisting pyroxenes for MgO/(MgO + FeO) and TiO₂, but not for Cr₂O₃. Secondary-textured micas do not show correlations with coexisting pyroxenes for any elements.The ‘primary-metasomatic’ micas described by Harte and Gurney (1975) and metasomatic and other micas described by Boettcher et al. (1979) and Boettcher and O'neil (1979) are richer in FeO and TiO₂ than the present primary-textured micas, and are attributed to crystallization from fractionated fluids.     

Thermal expansion and dehydroxylation of phengite micas

Phengite samples (2M ₁ and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray diffraction. The measured volume thermal expansion of 2M ₁ phengite (<α _V> ≈ 36.6 × 10⁻⁶ K⁻¹) was systematically greater than <α _V> of the 3T polytype (≈33.3 × 10⁻⁶ K⁻¹). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed in 2M ₁, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate phase derived from 2M ₁ is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H₂O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not proceed to completion. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Surface microtopography of metamorphic white micas

Gold decoration and platinum-carbon-shadowing techniques of electron microscopy have been applied to study the surface microtopographs of noncleavage (001) faces of porphyroblastic white micas and sericite crystals collected from the chlorite, biotite, and garnet zones in the Shiragayama area in the Sanbagawa metamorphic terrain, Shikoku, Japan. Two different types of surface microtopograph have been observed; parallel step system and saw-tooth step system. The former is interpreted as representing the surface microtopographs formed either by growth or under near equilibrium conditions, the latter either by dissolution or under rigorous kinetic conditions. The former has been observed on porphyroblastic white micas occurring in the middle portion of each metamorphic zone and on all sericite crystals, the latter on most sericites and on porphyroblastic white micas occurring along the boundaries of the neighboring metamorphic zones. It is suggested that the observed variations in the surface microtopographic characteristics are due to Ostwald ripening during a kind of sintering process in which interstitial water plays an essential role, and that the drastic changes along the metamorphic zone boundaries are due to dehydration reactions. Retrogressive metamorphism is considered to have an almost negligible effect, if any, upon the surface microtopographs.     

On micas from magmatic and metamorphic rocks

Micas from magmatic and metamorphic rocks differ from one another in chemical composition and in trace element content. The chemical composition of micas is discussed in relation to their occurrence, paragenesis and sequence of crystallization. On the basis of previous studies of the relationship between the physical properties and the chemical composition of 34 chemically analysed micas, reliable physical methods have been established which permit identification of different mica varieties in the same rock. Structural formulae and trace element content of micas from basic and granitic rocks, as well as from skarns, schists, ortho- and paragneisses are discussed. The relationship between the components of the tetrahedral and octahedral layers and of the interlayer are illustrated as ratios. Poorly differentiated, hybrid and metasomatic rocks often contain more than one variety of mica. Some prophyritic basalts and lamprophyres contain an early phlogopite which is paragenetically related to pyroxene phenocrysts and late biotite which occurs in the groundmass and in the fractures as a result of the crystallization of residual magma. The biotitemuscovite assemblage was observed in granodiorites, quartz-monzonites, schists and gneisses. In the albite-K-feldspar granites, muscovite predominates and the biotite is usually altered. The chemical composition of micas from metamorphic rocks depends on the grade of metamorphism and on the nature of associated minerals. The biotite from paragneisses contains considerable quantities of octahedral alumina. Pre-metamorphic micas show variable deficiencies of the (OH, F) group. The micas are useful minerals in determining the degree of differentiation and subsequent alteration of igneous rocks. The present study was carried out on the basis of 34 recent complete chemical analyses andca 100 X-ray fluorescence analyses. Dedicated to Professor Dr.Carl W. Correns on the occasion of his 70th birthday.     

Dehydration and diffusion of radiogenic argon in micas

There is a close analogy between dehydration and loss of argon curves obtained by laboratory heating of muscovite (figs. 2 and 3). Potassium atoms lie in potential cleavage planes of the muscovite. When K⁴⁰ atoms disintegrate the resulting argon atoms are driven into the mica structure by recoil, except those whose paths make a small angle with the cleavage plane. These remain in the "unstable zone" and may escape from the muscovite. This introduces an element of uncertainty in K/A absolute age figures from mica. -- F. E. Ingerson.     

Structures of micas synthesized at high pressures and temperatures

Thermal karst, exposed in mine shafts, has been significantly developed in the commercial stratum of oil shales. Geochemical investigations in this karst have for the first time established the secondary changes in the kerogen of the oil shales, namely, an anomalous content of bituminoid A + C, the presence of processes of aromatization, the significant amount of hydrocarbon compounds and humic acids, and changes in the elemental composition of the residual organic matter (in particular, an increase in the C/H ratio). The secondary changes suggest that a favorable geochemical and thermobaric environment existed in the thermal karst for intense bitumen formation. The results obtained are of interest in the study of the kerogen of oil shales as a possible parental source for oil and gas. —Authors.     

Chemical composition and physical properties of lithium-iron micas from the Krušné hory Mts. (Erzgebirge)

Compositions of natural lithium-iron micas are approximated best by the sidero-phyllite-polylithionite join. These micas contain little or no magnesium and manganese. Their octahedral sheets contain close to two trivalent cations (mainly aluminum) in small crystallographic sites and a variable quantity of lithium and R⁺² (mainly iron) in large sites. Octahedral vacancies are situated mostly in large sites. Lithium and R⁺² approach a 44 replacement relationship in micas with octahedral occupancy close to six. Lithium and fluorine show a good positive correlation (small excess of fluorine over lithium), which indicates a crystallochemical association between them. There is a less distinct positive correlation between lithium and R⁺⁴.Based on simplifications, a calculation shows that about two-thirds of octahedral vacancies are caused by substitutions within the octahedral sheet, one-third, by tetrahedral substitutions. Different methods of calculating the crystallochemical formula yield slightly different numbers of octahedral vacancies, but do not affect the mica's position in plots of physical parameters against composition. If a crystallochemical formula is calculated from analysis of a mica contaminated with quartz, topaz, or feldspar, the apparent number of octahedral vacancies increases; such a formula exhibits unusual behavior in composition plots.     

Cleavage mechanism and surface chemical characterization of phengitic Muscovite and Muscovite as constrained by X-Ray Photoelectron Spectroscopy

Freshly cleaved (001) natural muscovite was chemically characterized by X-ray Photoelectron Spectroscopy (XPS). The investigated muscovite crystals have composition ranging from pure end member to phengitic muscovite. XPS provides experimental evidence that the chemical composition of the near-surface region differs from the bulk. This difference is not due to analytical problems, but to extreme surface sensitivity of XPS. Depth profiling was also carried out after tilting of 60° or 70° of the sample along an axis perpendicular to [001]. After tilting of phengitic muscovite, the concentration of Al increases and of K decreases, and therefore Al is the topmost monolayer. Repulsion between the octahedral and tetrahedral sheets, stacking faults or more probably interlayered phases (gibbsite-like) are responsible for the cleavage. Possible perturbations of surface chemistry due to X-ray radiation and Ar⁺ ion sputtering have also been investigated. X-ray radiation is negligeable, but Ar⁺ ion sputtering introduces important structural modifications, perturbation of the chemical environment of elements and minor chemical variations. Received: 13 November 1996 / Revised, accepted: 21 March 1997     

An experimental study of alkali metal distributions in feldspars and micas

K and Rb distributions between aqueous alkali chloride vapour phase (0.7 molar) and coexisting phlogopites and sanidines have been investigated in the range 500 to 800°C at 2000 kg/cm² total pressure.Complete solid solution of RbMg₃AlSi₃O₁₀(OH)₂ in KMg₃AlSi₃O₁₀(OH)₂ exists at and above 700°C. At 500°C a possible miscibility gap between approximately 0.2 and 0.6 mole fraction of the Rb end-member is indicated.Only limited solid solution of Rb AlSi₃O₈ in KAlSi₃O₈ has been found at all temperatures investigated.Distribution coefficients, expressed as $\text{K}{}_{\mathrm{d}}\text{= (}\text{Rb/K}\text{)}$ in solid/(Rb/K) in vapour, are appreciably temperature-dependent but at each temperature are independent of composition for low Rb end-member mole fractions in the solids. The determined $\text{K}{}_{\mathrm{D}}$ values and their approximate Rb end-member mole fraction ($\text{X}{}_{\mathrm{RM}}$) ranges of constancy are summarized as follows: (°C)$\text{T}$$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Phlog/Vap.}}$$\text{X}{}_{\mathrm{RM}}$$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Sandi/Vap.}}$$\text{X}{}_{\mathrm{rm}}$

     

14.

Volatile contents of phlogopite micas from South African kimberlite     

Dean W. Matson  David W. Muenow  Michael O. Garcia 《Contributions to Mineralogy and Petrology》1986,93(3):399-408

Phlogopite micas from nodules in South African kimberlites were analyzed for major elements with the electron microprobe and for volatile contents by high temperature mass spectrometry. The micas are from primary- (deformed) and secondary- (undeformed) textured grains in perodotite xenoliths, glimmerites, MARID (mica-amphibole-rutile-ilmenite-diopside) suite nodules and a mica megacryst. The major element and volatile contents of micas exhibiting these modes of occurrence overlap to a greater extent than indicated in previous studies. Concentrations of volatile species occupying structurally defined crystallographic sites (H₂O, F, Cl) are greater for many of the micas than predicted on the basis of the mica formula, particularly for the glimmerite and MARID suite samples. A correlation exists between micas with tetrahedral and octahedral cation deficiencies and those with excess H₂O, F and Cl. Substitution of H⁺ for tetrahedral and possibly octahedral cations may be responsible for the excess H₂O in these micas. Except for one sample, the major element and volatile data for the peridotite, glimmerite and MARID suite micas indicate that they crystallized at oxygen fugacities below the quartz-fayalite-magnetite buffer. F and K₂O are in the correct proportion in the micas to provide the source for these elements in alkali basalts, but not in mid-ocean ridge basalts. Kaersutite amphibole is a more likely source of potassium and fluorine in mid-ocean ridge basalts.     

15.

Lithium in the interlayer space of synthetic trioctahedral micas     

J.-L. Robert  M. Volfinger  J.-N. Barrandon  M. Basutu 《Chemical Geology》1983,40(3-4):337-351

This paper gives several new and strong arguments in favour of the possibility of fixation of anhydrous lithium in the interlayer space of trioctahedral potassium micas. From the chemical viewpoint Li⁺ can replace K⁺, but is located out of the alkaline cation site; it enters pseudo-octahedral cavities limited by the triangular bases of two aluminous tetrahedra of two consecutive sheets. The solubility limit of Li⁺ in the interlayer is a function of the Al^IV content of the mica. It is given by the relation (Li/Li + K)_max = 2[(Al/Si + Al)^IV]². In both micas investigated — phlogopite, KMg₃(Si₃Al)O₁₀(OH)₂, and eastonite, K(Mg_2.5Al_1.5)(Si_2.5Al_1.5)O₁₀(OH)₂ — there is a remarkable agreement between the calculated values of the solubility limits and those measured by exchange reactions with hydrothermal solutions, at 600°C, 2 kbar. In high-Al micas, the interlayer Li content can be very important, with about one-third of K⁺ replaced by Li⁺.

The fixation of Li⁺ according to this model provokes a strong flattening of the interlayer (strong decrease of the reticular distance d₀₀₅) and a slight increase of the reticular distance d₀₆₀. Infra-red (IR) absorption spectrometry shows that vacant K⁺ sites are created when Li⁺ enters the interlayer; one observes low-frequency OH stretching bands attributed to OH dipoles lying towards these empty sites. Fixation of Li⁺ does not provoke any modification of the IR spectra in the region 1200-300 cm⁻¹, indicating that Li⁺ is really out of the sheet. For both cell dimensions and IR spectra, a comparison is made with “ordinary” lepidolites, having Li⁺ in the octahedral sheet; it provides a guide for the distinction between the two species of Li-bearing micas.     

16.

Mixing properties and stability of jadeite-acmite pyroxene in the presence of albite and quartz     

Jun Liu  Steven R. Bohlen 《Contributions to Mineralogy and Petrology》1995,119(4):433-440

The stability of synthetic jadeite-acmite pyroxene coexisting with albite and quartz has been determined at 600, 700, and 900° C. The end-member reaction: albite = jadeite + quartz has been determined to lie between 1.67 and 1.70 GPa at 600° C, 1.88 and 1.90 GPa at 700° C, and 2.44 and 2.48 GPa at 900° C. Jd₇₈Acm₂₂ + quartz is stable above 1.58, 1.78, and 2.33 GPa at 600, 700, and 900° C, respectively. Jd₆₁Acm₃₉ + quartz is stable above 1.47, 1.67, and 2.18 GPa at 600, 700, and 900° C, respectively. Addition of as much as 40% of acmite component in jadeite extends pyroxene stability by less than 300 MPa at 900° C. Unit-cell parameters measured for synthetic jadeite-acmite pyroxenes indicate linear volume-composition relations. The data are consistent with ideal mixing in jadeite-acmite solutions.     

17.

The relation between c dimension and exchange components in micas     

Embaie Asmerom Ferrow 《Mineralogy and Petrology》1990,43(1):23-35

Summary A number of micas of varying compositions and polytypism have been selected from the literature for multiple linear regression analysis. The c dimension in micas is found to depend on the sizes of the interlayer cation, d_i, and tetrahedral cation, d_t, as well as on the hydroxyl content, [OH]. The regression equation obtained: c_r = 5.415 + 0.071[OH] + 2.098d_i + 2.335d_t with R² = 90.5%, shows that the three variables affect the c-axis dimension in the order d_t > d_i [OH]. Addition of 2- and 3-layer polytypes to the regression analysis reduces R² to 87.2%. Application of the regression analysis to synthetic Al-rich biotites from the literature shows that the amount of [A1^IVA1^VI]_1Y[Fe²⁺, MgSi]_–1y in solid solution is limited and always less than [A1^VIO]_1z[Fe²⁺, MgOH]_–1z (i.e. 0.35 > y z). The maximum value of the vector y in solution is slightly higher than that reported for natural Al-rich biotites.

---

Die Beziehung zwischen der Gitterkonstante c und den Austauschkomponenten in Glimmern

Zusammenfassung Eine Anzahl von Glimmern unterschiedlicher Zusammensetzung und Polytypie wurde aus der Literatur für eine multiple lineare Regressionsanalyse ausgewählt. Es stellte sich heraus, dass in Glimmern die Gitterkonstante c von den Grössen des Zwischenschicht-Kations d_i und des tetraedrischen Kations d_t abhängt, ferner vorn Hydroxylgehalt, [OH]. Die erhaltene Regressionsgleichung c_r = 5,415 + 0,071 [OH] + 2,098d_i + 2,335d_t mit R² = 90,5% zeigt, dass die drei Variablen die Grösse der c-Achse in der Reihenfolge d_t > d_i [OH] beeinflussen. Der Einschluss von 2- und 3-Schicht Polytypen in die Regressionsanalysen verkleinert R² auf 87,2%. Die Anwendung der Regressionsanalysen auf synthetische Al-reiche Biotite aus der Literatur zeigt, der Betrag von [A1^IVA1^VI]_1y[Fe²⁺, MgSi]_–1y beschränkt und immer kleiner als [A1^VIO]_1z[Fe²⁺, MgOH]_–1z (mit 0,35 < y z) ist. Der Maximalwert des Vektors y in Lösung ist etwas grösser als jener, der für natürliche Al-reiche Biotite angegeben wurde.

---

---

With 1 Figure     

18.

Evolution of zoned micas and associated silicates in the Oka carbonatite   总被引：1，自引：0，他引：1  

Jadvyga Rimsaite 《Contributions to Mineralogy and Petrology》1969,23(4):340-360

Thin section study of the Oka carbonatite revealed the presence of zoned pyroxene, amphibole, mica and carbonate which were studied by means of electron probe line scans to establish the relationship between optical zoning and chemical composition. The zoned micas were studied in more detail by means of optical, X-ray, classical chemical, spectrographic and electron probe point counting analyses. The results indicated two- to ten-fold variations in ionic proportions of titanium, iron and aluminium, and lesser variations in concentrations of magnesium, silicon and potassium in six different mica zones.Distribution coefficients of Si, Al, Fe, Fe, Ti and Mg in adjacent mica zones indicate that, with the exception of ferrous and ferric iron which show a straight line relation, the relationship between the major constituents is more complex. Iron and magnesium exhibit opposite trends, modified by prominent variations in aluminium and titanium. Gradual variations within the zones are accounted for by gradual differentiation of the magma. The abrupt changes in chemical composition between the zones, coinciding with optical boundaries, indicate sudden changes in the environmental conditions, resulting from crystallization of associated minerals and periodic emplacement of certain elements into magma.High Fe and high Ti-Al zones are used as markers for correlating cogenetic zones in pyroxene, amphibole and mica, and for establishing periods of crystallization of Fe-Ti ore minerals.     

19.

Fe---Mg partitioning between coexisting garnet and phengite at high pressure, and comments on a garnet-phengite geothermometer   总被引：17，自引：0，他引：17  

T. H. Green  P. L. Hellman 《Lithos》1982,15(4):253-266

The Fe---Mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite (mg = 67) in the presence of quartz and water at pressures of 20–35 kb and temperatures of 800–1000°C. Compositions of coexisting garnet and mica indicate a linear relation between both the InK_D((Fe/Mg) garnet/(Fe/Mg) phengite) and temperature, and InK_D and pressure in the above P.T range. This Fe---Mg exchange reaction between garnet and phengite is shown to be dependent on the Ca-content of the garnet, and on the mg number of the bulk composition. These two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46, and by using a synthetic basaltic composition. The overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems, respectively. However, because of non-ideality in the exchange reaction, this geothermometer should not be used in any practical application outside the composition ranges studied. Also, careful consideration of the presence of Fe³⁺ in phengite must be made. If the Fe³⁺ content of the natural phengite is unknown, then the temperatures obtained will be maximum temperatures only.     

20.

Magnetic behavior of trioctahedral micas with different octahedral Fe ordering     

Stefano Pini  Maria Franca Brigatti  Marco Affronte  Daniele Malferrari  Augusto Marcelli 《Physics and Chemistry of Minerals》2012,39(8):665-674

This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition ^IV(Si_2.76Al_1.24) ^VI(Al_0.64Mg_0.72 $ {\text{Fe}}_{1.45}^{2 + } $ Mn_0.03Ti_0.15) (K_0.96Na_0.05) O_10.67 (OH)_1.31 Cl_0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition ^IV(Si_3.14Al_0.86)^VI(Al_0.75Mg_0.01 $ {\text{Fe}}_{1.03}^{2 + } $ $ {\text{Fe}}_{1.03}^{3 + } $ Mn_0.01Ti_0.01Li_1.09) (K_0.99Na_0.01) O_10.00 (OH)_0.65F_1.35]. Samples differ for Fe ordering in octahedral sites, Fe²⁺/(Fe²⁺?+?Fe³⁺) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6?K in TK, where Fe is disordered in all octahedral sites, and 8?K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites.     

(°C)T	$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Phlog/Vap.}}$	$\text{X}{}_{\mathrm{RM}}$	$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Sanid/Vap.}}$	$\text{X}{}_{\mathrm{RM}}$
500	$\text{0.64 ± 0.11}$	0–0.2	$\text{0.17 ± 0.04}$	0–0.07
700	$\text{1.11 ± 0.11}$	0–0.2	$\text{0.33 ± 0.04}$	0–0.1
800	$\text{1.28 ± 0.03}$	0–0.2	$\text{0.45 ± 0.06}$	0–0.1

Volatile contents of phlogopite micas from South African kimberlite

Phlogopite micas from nodules in South African kimberlites were analyzed for major elements with the electron microprobe and for volatile contents by high temperature mass spectrometry. The micas are from primary- (deformed) and secondary- (undeformed) textured grains in perodotite xenoliths, glimmerites, MARID (mica-amphibole-rutile-ilmenite-diopside) suite nodules and a mica megacryst. The major element and volatile contents of micas exhibiting these modes of occurrence overlap to a greater extent than indicated in previous studies. Concentrations of volatile species occupying structurally defined crystallographic sites (H₂O, F, Cl) are greater for many of the micas than predicted on the basis of the mica formula, particularly for the glimmerite and MARID suite samples. A correlation exists between micas with tetrahedral and octahedral cation deficiencies and those with excess H₂O, F and Cl. Substitution of H⁺ for tetrahedral and possibly octahedral cations may be responsible for the excess H₂O in these micas. Except for one sample, the major element and volatile data for the peridotite, glimmerite and MARID suite micas indicate that they crystallized at oxygen fugacities below the quartz-fayalite-magnetite buffer. F and K₂O are in the correct proportion in the micas to provide the source for these elements in alkali basalts, but not in mid-ocean ridge basalts. Kaersutite amphibole is a more likely source of potassium and fluorine in mid-ocean ridge basalts.     

Lithium in the interlayer space of synthetic trioctahedral micas

This paper gives several new and strong arguments in favour of the possibility of fixation of anhydrous lithium in the interlayer space of trioctahedral potassium micas. From the chemical viewpoint Li⁺ can replace K⁺, but is located out of the alkaline cation site; it enters pseudo-octahedral cavities limited by the triangular bases of two aluminous tetrahedra of two consecutive sheets. The solubility limit of Li⁺ in the interlayer is a function of the Al^IV content of the mica. It is given by the relation (Li/Li + K)_max = 2[(Al/Si + Al)^IV]². In both micas investigated — phlogopite, KMg₃(Si₃Al)O₁₀(OH)₂, and eastonite, K(Mg_2.5Al_1.5)(Si_2.5Al_1.5)O₁₀(OH)₂ — there is a remarkable agreement between the calculated values of the solubility limits and those measured by exchange reactions with hydrothermal solutions, at 600°C, 2 kbar. In high-Al micas, the interlayer Li content can be very important, with about one-third of K⁺ replaced by Li⁺.

The fixation of Li⁺ according to this model provokes a strong flattening of the interlayer (strong decrease of the reticular distance d₀₀₅) and a slight increase of the reticular distance d₀₆₀. Infra-red (IR) absorption spectrometry shows that vacant K⁺ sites are created when Li⁺ enters the interlayer; one observes low-frequency OH stretching bands attributed to OH dipoles lying towards these empty sites. Fixation of Li⁺ does not provoke any modification of the IR spectra in the region 1200-300 cm⁻¹, indicating that Li⁺ is really out of the sheet. For both cell dimensions and IR spectra, a comparison is made with “ordinary” lepidolites, having Li⁺ in the octahedral sheet; it provides a guide for the distinction between the two species of Li-bearing micas.     

Mixing properties and stability of jadeite-acmite pyroxene in the presence of albite and quartz

The stability of synthetic jadeite-acmite pyroxene coexisting with albite and quartz has been determined at 600, 700, and 900° C. The end-member reaction: albite = jadeite + quartz has been determined to lie between 1.67 and 1.70 GPa at 600° C, 1.88 and 1.90 GPa at 700° C, and 2.44 and 2.48 GPa at 900° C. Jd₇₈Acm₂₂ + quartz is stable above 1.58, 1.78, and 2.33 GPa at 600, 700, and 900° C, respectively. Jd₆₁Acm₃₉ + quartz is stable above 1.47, 1.67, and 2.18 GPa at 600, 700, and 900° C, respectively. Addition of as much as 40% of acmite component in jadeite extends pyroxene stability by less than 300 MPa at 900° C. Unit-cell parameters measured for synthetic jadeite-acmite pyroxenes indicate linear volume-composition relations. The data are consistent with ideal mixing in jadeite-acmite solutions.     

The relation between c dimension and exchange components in micas

Summary A number of micas of varying compositions and polytypism have been selected from the literature for multiple linear regression analysis. The c dimension in micas is found to depend on the sizes of the interlayer cation, d_i, and tetrahedral cation, d_t, as well as on the hydroxyl content, [OH]. The regression equation obtained: c_r = 5.415 + 0.071[OH] + 2.098d_i + 2.335d_t with R² = 90.5%, shows that the three variables affect the c-axis dimension in the order d_t > d_i [OH]. Addition of 2- and 3-layer polytypes to the regression analysis reduces R² to 87.2%. Application of the regression analysis to synthetic Al-rich biotites from the literature shows that the amount of [A1^IVA1^VI]_1Y[Fe²⁺, MgSi]_–1y in solid solution is limited and always less than [A1^VIO]_1z[Fe²⁺, MgOH]_–1z (i.e. 0.35 > y z). The maximum value of the vector y in solution is slightly higher than that reported for natural Al-rich biotites.

---

Die Beziehung zwischen der Gitterkonstante c und den Austauschkomponenten in Glimmern

Zusammenfassung Eine Anzahl von Glimmern unterschiedlicher Zusammensetzung und Polytypie wurde aus der Literatur für eine multiple lineare Regressionsanalyse ausgewählt. Es stellte sich heraus, dass in Glimmern die Gitterkonstante c von den Grössen des Zwischenschicht-Kations d_i und des tetraedrischen Kations d_t abhängt, ferner vorn Hydroxylgehalt, [OH]. Die erhaltene Regressionsgleichung c_r = 5,415 + 0,071 [OH] + 2,098d_i + 2,335d_t mit R² = 90,5% zeigt, dass die drei Variablen die Grösse der c-Achse in der Reihenfolge d_t > d_i [OH] beeinflussen. Der Einschluss von 2- und 3-Schicht Polytypen in die Regressionsanalysen verkleinert R² auf 87,2%. Die Anwendung der Regressionsanalysen auf synthetische Al-reiche Biotite aus der Literatur zeigt, der Betrag von [A1^IVA1^VI]_1y[Fe²⁺, MgSi]_–1y beschränkt und immer kleiner als [A1^VIO]_1z[Fe²⁺, MgOH]_–1z (mit 0,35 < y z) ist. Der Maximalwert des Vektors y in Lösung ist etwas grösser als jener, der für natürliche Al-reiche Biotite angegeben wurde.

---

---

With 1 Figure     

Evolution of zoned micas and associated silicates in the Oka carbonatite

Thin section study of the Oka carbonatite revealed the presence of zoned pyroxene, amphibole, mica and carbonate which were studied by means of electron probe line scans to establish the relationship between optical zoning and chemical composition. The zoned micas were studied in more detail by means of optical, X-ray, classical chemical, spectrographic and electron probe point counting analyses. The results indicated two- to ten-fold variations in ionic proportions of titanium, iron and aluminium, and lesser variations in concentrations of magnesium, silicon and potassium in six different mica zones.Distribution coefficients of Si, Al, Fe, Fe, Ti and Mg in adjacent mica zones indicate that, with the exception of ferrous and ferric iron which show a straight line relation, the relationship between the major constituents is more complex. Iron and magnesium exhibit opposite trends, modified by prominent variations in aluminium and titanium. Gradual variations within the zones are accounted for by gradual differentiation of the magma. The abrupt changes in chemical composition between the zones, coinciding with optical boundaries, indicate sudden changes in the environmental conditions, resulting from crystallization of associated minerals and periodic emplacement of certain elements into magma.High Fe and high Ti-Al zones are used as markers for correlating cogenetic zones in pyroxene, amphibole and mica, and for establishing periods of crystallization of Fe-Ti ore minerals.     

Fe---Mg partitioning between coexisting garnet and phengite at high pressure, and comments on a garnet-phengite geothermometer

The Fe---Mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite (mg = 67) in the presence of quartz and water at pressures of 20–35 kb and temperatures of 800–1000°C. Compositions of coexisting garnet and mica indicate a linear relation between both the InK_D((Fe/Mg) garnet/(Fe/Mg) phengite) and temperature, and InK_D and pressure in the above P.T range. This Fe---Mg exchange reaction between garnet and phengite is shown to be dependent on the Ca-content of the garnet, and on the mg number of the bulk composition. These two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46, and by using a synthetic basaltic composition. The overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems, respectively. However, because of non-ideality in the exchange reaction, this geothermometer should not be used in any practical application outside the composition ranges studied. Also, careful consideration of the presence of Fe³⁺ in phengite must be made. If the Fe³⁺ content of the natural phengite is unknown, then the temperatures obtained will be maximum temperatures only.     

Magnetic behavior of trioctahedral micas with different octahedral Fe ordering

This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition ^IV(Si_2.76Al_1.24) ^VI(Al_0.64Mg_0.72 $ {\text{Fe}}_{1.45}^{2 + } $ Mn_0.03Ti_0.15) (K_0.96Na_0.05) O_10.67 (OH)_1.31 Cl_0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition ^IV(Si_3.14Al_0.86)^VI(Al_0.75Mg_0.01 $ {\text{Fe}}_{1.03}^{2 + } $ $ {\text{Fe}}_{1.03}^{3 + } $ Mn_0.01Ti_0.01Li_1.09) (K_0.99Na_0.01) O_10.00 (OH)_0.65F_1.35]. Samples differ for Fe ordering in octahedral sites, Fe²⁺/(Fe²⁺?+?Fe³⁺) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6?K in TK, where Fe is disordered in all octahedral sites, and 8?K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites.